Put the summary of the bug you're having here

What I expected to happen was...:
I thought my worlds would load normally.

What actually happened was...:
The small pictures in the worlds menu were grey and unknown was somewhere within the description of each world, under the date the world was last played. When I try to load a world, it loads and I see the world for a split second but then the game closes and error texts show up. I tried to turn off the game and load the world again and the same thing happened. I started a new world, it loaded, I left the world and then the same thing happened to the new world I just started, it closes and error texts come up. I am unable to open any worlds from my minecraft pc edition, whether new or old.

Steps to Reproduce:
1. Open the launcher
2. Start the game
3. GO to worlds
4. Start playing one of the worlds
5. It will close and error texts will show up

(Launcher is on version 1.6.61